US2122222A - Device for automatic interruption of radiation - Google Patents

Description

June 28, 1938. A. w. VINGERHOETS 2,122,222

DEVICE FOR AUTOMATIC INTERRUPTION OF RADIATION Filed Oct. 12, 1934 T! g 6! nwmrm 1J d/ 4. mm/us W/LHELMUS V/NGERHOEB fir Patented June 28, 1938 UNITED STATES DEVICE FOR AUTOMATIC INTERRUPTION F RADIATION Antonius Wilhelmus Vingerhoets, Eindhoven,

Netherlands, assignor to N. V. Philips Gloeilampenfabrieken, Elndhoven, Netherlands Application October 12, 1934, Serial No. 748,151 In Germany November 6, 1933 7 Claims.

The present invention relates to devices for the automatic interruption of the radiation caused by X-rays, gamma rays etc., after such radiation has produced a desired effect, for instance, a sufficient exposure (blackening) of a photographic plate. My invention will be described in its application to the taking of X-ray exposures.

It is known that X-rays cause the ionization of air, and that ionization chambers, placed behind the object and the photographic plate and ionized by the X-rays penetrating through same, can be used for measuring the eiiect of the rays.

It has already been proposed to use the loss of voltage of a charged condenser in combination with such an ionization chamber to automaticab 1y interrupt an X-ray exposure, when a sufficient exposure of the photographic plate or film has taken place. Such a device has been described in the Netherlands Patent 29,448, in which an ionization chamber is subjected to the effect of X-rays during the exposure, and the ionization in the chamber causes a pare-charged condenser to lose its charge. She condenser biases the grid of a triode, the plate circuit of which controls, by means of a switch, the operating circuit of the X-ray tube; whereby upon loss of charge of the condenser, the increased plate current of the triode causes the actuation of the switch and the interruption of the operating circult of the X-ray tube.

Such arrangement, however, behaved too erratically to be of practical use and too short or too long exposures were obtained under apparently identical circumstances.

The lack of reliability of the arrangement was primarily due to the various sources of leakage outside of the ionization chamber, the value of the leakage currents greatly depending, as a rule, upon accidental and secondary influences, such as the moisture content of the air, surface conditions of the leads, etc., resulting in great variations in the discharge time of the condenser.

The present invention has for its objectto avoid such deleterious and accidental influences of leakage outside of the ionization chamber, and to thereby provide a device of this kind which is reliable and accurate.

According to the invention, instead of providing a separate charging circuit for the condenser, the condenser is charged through the vacuum tube to the grid of which it is connected, thereby avoiding the lead connecting the grid and the corresponding electrode of the condenser to a terminal of the charging source. This eliminates an important source of leakage.

Furthermore, instead of providing a separate condenser the ionization chamber itself is formed as to constitute the condenser. For this purpose the electrodes of the ionization chamber are given a sufficiently large area as to give the required capacity. This causes the elimination oi further leakage sources. This increased insulating resistance makes it possible to use a much smaller capacity, which increases the sensitlveness of the adjustment. The grid being solely connected to one electrode of the ioniza== tion chamber, the connecting lead between these members can be made quite short and can be hermetically sealed, which decreases the surface leakage of this lead to a minimum.

Preferably the ionization chamber is rigidly connected to the vacuum tube by a tube of insulating inateriai which encloses the connecting lead between the grid and the ionization chamher, which tube is preferably exhausted. The ionization chamber and electron relay may thereby constitute a single device.

To further reduce the leakage current the insulating member supporting the two electrodes of the ionization chamber can be intercepted by a conductive zone to which zone a negative potential may be applied, which potential may be that 01' the grid or may be still lower. This permits compensation of the leakage between the grid and the cathode (which is positive with respect to the grid).

If desired, the same source can be used to charge the ionization chamber (condenser) and to supply the plate current for the electron relay. Such source may be, for instance, a rectifled alternating current, which is preferably filtered, and across which is connected a potentiometer, from which the various desired voltages can be tapped off.

To improve the constancy of the voltage a gas discharge path may be provided. As such a discharge path has a definite ignition voltage, the voltage applied on the device does not vary when the voltage of the supply mains shows an incidental increase.

In the case when, according to the invention, the condenser is charged through the electron relay, instead of a rectified current an alternating voltage may be used.

Various further novel features and objects of my invention will appear as the specification progresses.

In order that the invention may be clearly understood and readily carried into eil'ect, it will now be described more fully by way of example with reference to the accompanying drawing, in which:

Figure 1 is a circuit diagram of a device arranged and connected according to the prior art.

Fig. 2 is a circuit diagram showing the arrangement and connection or a device according to one embodiment of my invention.

Fig. 3 is a circuit arrangement showing a preferred form of my invention, also showing diegrammatically a device combining the ionization chamber and the electron relay in a single unit.

Fig. 4 is a fractional and sectionized side view of the combined ionization chamber and electron relay.

Referring to Figure 1, there is provided an ionization chamber I to which is connected in parallel, through leads 60 and iii, a condenser 2. The condenser 2 can be charged from a suitable D. C. current source 3, a switch 4 being provided in the negative branch 65 of the charging circuit. A high vacuum triode 6, having an lncandescent cathode 5, a grid 8, and a plate I0, is connected with its grid 8 to the negative electrode of the condenser 2 and to the negative electrode 62 of ionization chamber i. The plate circuit of tube 6 includes a suitable current source 9, a device H to control the energlzation of the X-ray tube, and a switch 1, to open or close this circuit. The device H may for example consist of a magnetic relay, the coil of which is connected in the plate circuit, and the armature of which controls the primary circuit of the high voltage transformer of the X-ray tube. The circuits for such a control by device ll, being obvious to one skilled in the art, have been omitted for the sake of simplicity. The same applies to the heating circuit of the oathode 5.

The negative electrode 62 of the ionization chamber I, as stated, is connected with the grid 8 of the tube 6, whereas its positive electrode 63, together with the positive side of source 3 and of condenser 2, is connected at Bl to the plate circuit or the tube 6.

Before taking the X-ray exposure the condenser is charged to the proper voltage by closure of the switch I. For taking the exposure the cathode 5 is brought to its operating temperature and the switch 1 is closed, and after an intermediate closure of switch 4 the latter is opened simultaneously with the closure of the main operating switch of the X-ray tube.

The grid 8, being initially negatively biased with respect to the cathode 5, prevents altogether, or at least to a great extent, a current flow through the plate circuit of the tube, thus keeping the relay switch ll closed. As the X-ray exposure progresses, ionization in the ionization chamber, establishes a discharge circuit for condenser 2 and the condenser gradually discharges through this circuit.

This reduces the negative bias of grid 8 and increases the plate current, and after a given time interval, which is predetermined by the proper selection of the various factors involved, and which corresponds to the desired blackening of the photographic film, the grid bias assumes a value which allows a sufilciently high plate current to pass through the coil of relay Ii to cause the relay to open the operating circuit of the X-ray tube, thereby terminating the exposure.

In such an arrangement, at the lead 85 connecting the condenser with the negative side of the charging source, and at the leads Bil-6| interconnecting the condenser with the ionization chamber, a considerable and uncontrollably varying leakage may occur.

This source of leakage can be eliminated by an arrangement according to the invention as shown in Fig. 2.

Referring to Fig. 2 the changes, compared to Figure 1, include two steps: The first step comprises the elirnination of the connection between the grid and the charging source. The condenser 2, indicated in dotted lines, instead of being charged directly, is charged through the electron tube 6. A. two-position switch [2 is provided instead of the two switches 4 and l. The condenser 2 is charged through the following circuit: Positive terminal of source 3, condenser 2, grid 8, cathode 5, switch l2, contact 40, back to the negative terminal of source 3. The heating circuit of the cathode (not shown) being closed and the grid being positive with regard to the cathode, a current of sufiicient magnitude flows through the plate circuit to keep the relay H open.

When taking the exposure the switch i2 is moved into its right-hand position to engage contact 4|. This connects the cathode 5 to the positive side of the condenser 2, which renders the grid negatively biased in respect to the oathode 5. This in turn cuts off the plate current and deenergizes relay H, thus closing the operating circuit of the X-ray tube. During the exposure, the ionization chamber becoming conductive, the condenser 2 is gradually discharged and the potential of grid 8 becomes less negative in respect to the cathode 5, until finally the plate current becomes sufficiently large to actuate the relay and interrupt the exposure.

The second step to improve the arrangement consists in the omission of the condenser by using a properly designed ionization chamber as the condenser. For this purpose the two electrodes 62 and 63 of the ionization chamber are given suflicient area and they charge up and discharge in the same manner as has been just described for condenser 2.

By omitting the condenser and its corresponding leads, further sources of leakage are removed, and at the same time the device is greatly simplified. The greatly reduced leakage also makes it possible to use much smaller capacities, thereby increasing the sensitiveness and accuracy of the device.

In Fig. 3 the ionization chamber 13 and the electron relay I4 are connected into a single unit having, if desired, a common envelope. The envelopes of these devices consist as a. rule of vitreous material, and are interconnected by a short tube [5 of insulating material, preferably of vitreous material fused to the envelopes oi devices l3 and H.

The electrode I! of the ionization chamber and the grid l8 of the vacuum tube are interconnected by the connection I6 which is made as short as possible and is perfectly air-tightly sealed in the connecting tube l5. In this arrangement a common current source is assumed for the charging of the condenser and for the plate current of the tube M. This source supplies rectified and smoothed current from the ordinary alternating current network 56. The primary winding of a transformer 51 is connected to the network and its secondary winding i9 is in seiii) ries with a rectifier 20. The rectified current so obtained is smoothed by suitable filtering means, for instance by a condenser 38 connected across the series arrangement of the rectifier 20 and the winding 19.

Connected across the condenser 38 at 45 and 46 is a resistor 23 acting as a potentiometer, from which the various desired potentials are tapped off. The electrode 2| of the ionization chamber I3 is connected to the positive end 45 of theresistor 23, and the electrode I1 is connected to the grid 18 of the'electron relay M by means of lead I6. The end 46 of the resistor 23 is connected to the contact 66 of switch l2, whereas the contact 61 of the switch is connected to a. tap 24 of the resistance comparatively close to the negative end 45 of the resistor 23.

In this arrangement, the device for the control of the X-ray tube is not placed directly in the plate circuit of the electron tube I4, but is indirectly controlled thereby. For this purpose, the cathode 22 of the electron tube I4 is connected to the heel 39 of the switch I2, whereas its plate 21 is connected to one end 48 of a variable resistor 26, a portion of which can be short-circuited by means of sliding contact 58. The other end 4'? of resistance 26 is connected to a tap 25 of the potentiometer 23, which tap is positive with respect to tap 24, and is also connected to the cathode 28 of a gas=filled tube 29, the control electrode 30 of which is connected to the end M of resistance 26. Tube 29 is of the type acting with an arc discharge and is not ignited until the control electrode has exceeded definite potential.

A current source 3%, supplying an alternating or pulsating voltage, is provided in the plate cir cult of the gas-filled tube 29, which circuit also includes the coil 32 of an electromagnetic relay. The armature 3d of this relay controls the energization of the X-ray tube ill, for instance by closing the primary circuit of the high tension. transformer which supplies the operating voltage for the X-ray tube and is energized from the means 12.

The end 49 of resistance 26 is negative compared to the end 41, the potential difference between the two depending on the voltage drop in resistance 26 and thus on the value of the plate current of tube l4. When a large current flows through the plate circuit of tube l4, the voltage drop in resistance 26 is large and renders the control electrode 30 negative with respect to the cathode 28. When the plate current of tube I4 drops below a given value, the control electrode 30 sharply cuts off the current flow through the plate circuit of tube 29, which causes the dropping of armature 34.

To oppositely charge the electrodes of the ionization chamber 23, the switch I2 is brought into its right-hand position to engage contact 66. The cathode 22 is thereby rendered negative with respect to the grid l8, and the electrodes ii and 2! are charged up through the following charging circuit. From the positive end 45 of resistor 23 through electrode 2|, electrode ll, connecting lead it, grid 58, cathode 22, switch i2, contact 68 and negative end 45 of resistor 23.

The cathode 22 being negative with respect to the grid ill, a sufiiciently large plate current flows through tube M as to cause a voltage drop in resistance 26, which provides a sufiicient negative bias on control electrode 38 as to prevent current flow through the plate circuit of tube 29,

thereby keeping the coil :2 deenerglzed and the armature 34 open.

To take the exposurethe ionization chamber electrodes having been previously charged. as just explained-the switch I2 is moved to its lefthand position contacting with contact 61. This renders the cathode 22 positive with respect to the grid l5, and whereby the current flow through the plate circuit of the tube I4 is interrupted or at least greatly reduced. This, in turn, reduces the voltage drop in the resistor 26 and the bias of control electrode 30 becomes less negative (falling below its cut-off value) and thus a current passes through the plate circuit of tube 29, which energizes coil 32 and causes, by means of armature 34, the closure of the operating circuit of the X-ray tube, which starts the exposure.

As the exposure proceeds, the X-rays passing through the object and the photographic plate strike the ionization chamber ll, causing it to be ionized, which results in a gradual equalization of the charges on the electrodes i1 and 2. Thereby, the grid l8 loses its negative potential and the plate current through tube l4 gradually increases until it reaches a value which causes, in resistance 26, a voltage drop which is sufiiciently large to cause control electrode 30 to assume a sufliciently negative bias to cut oil the current flow through the plate circuit of tube 29, and by means of the relay, to interrupt the exposure.

The various factors are so selected and adjusted as to cause this interruption to take place at the instance when the exposure of the tube has produced the desired blackening of the photographic plate or film.

Fig. 4 illustrates a construction in which the ionization chamber l3 and the electron tube M are rigidly interconnected to form a single unit, the two being interconnected by means of a hollow tube 35. The envelopes of chamber 13 and tube M respectively, together with the tube 35, may constitute a single vitreous body.

To lengthen the leakage path and to improve the insulation, the ionization chamber is pro" vided with a neck 68 of which tube 35 may form an extension, and through which passes a re-en trant tubular portion 36 of considerable length. The tube 36 supports and is partly surrounded by a hood-shaped inner electrode l1 formed of foil, whereas the electrode 2i which is arranged substantially parallel to the chamber envelope is made of thin sheet metal, although it may be formed of a foil coating of the wall or a metal deposit thereon. The electrode 2| is supported from the envelope of chamber l3 by a wire 63 which also serves as a lead thereto. A portion of the envelope and the tube 36 form a very long leakage path between electrodes I! and 2!, which path is also interrupted by a metal ring 37.

Great care must be taken that the lead between the electrode i! of the ionization chamber and the grid E8 of the electron tube has a very high insulation so as to minimize all loss of electric charge. Therefore, the lead 55-55' is supported only at its two ends at 53 and 54. The lead 5555' may form a single lead or may be separate leads interconnected at l6. As a rule, I prefer to manufacture the ionization chamber and the electron tube separately and thereafter interconnect them by fusing the tube 35 to the respective envelopes of the ionization chamber l3 and the discharge tube H4.

The tube 35 is preferably sealed from both the ionization chamber I3 and the relay tube I4,

and is exhausted. However, the space within tube Il may also be interconnected with either the ionization chamber II or with the relay tube ll. In the latter case, of course, the tube cannot be exhausted before the two are iused together.

When the device is in operation, the metal ring ll may be connected to a negative potential, for

instance, to the end 46 of resistance 23 (Fig. 3).

whereby the leakage set up between the grid i8 and the ring 31 compensates for the loss of negative charge towards the cathode. The charge first lost by the leakage flow between the grid and the cathode is afterwards, when the grid potential increases, regained by the leakage ilow between the ring 31 and the grid. The ring 31 also prevents the condenser from being discharged along the wall of the ionization chamber.

While I have described my invention with respect to a specific application and have illustrated specific embodiments thereof, I do not wish to be limited thereto, but desire the appended claims to be construed as broadly as permissible in view of the prior art.

What I claim is:

1. A device for automatically determining the duration of irradiation, comprising a vacuum discharge tube having a cathode, a plate and a grid, said grid being adapted to control the plate current 01 the tube, switching means controlled by the plate current of said tube to initiate and terminate said irradiation, an ionization chamber having a wall adapted to be subjected to said irradiation and comprising at least two electrodes,

one of said electrodes being connected to said grid, 0. source of direct current, a circuit including said source, said chamber, grid and cathode,

. said source supplying through the cathode a negative charge to that chamber-electrode which is connected to the grid, means including a condenser connected in parallel with said ionization chamber to provide for a greater negative charge on said electrode, and means co-operating with said circuit to establish a predetermined decrease of the potential diflerence between the cathode and the other chamber-electrode, said negatively charged electrode discharging through the ionization chamber during a time interval which determines the duration of said irradiation.

2. A device for automatically determining the duration of irradiation, comprising a vacuum discharge tube having an incandesclble cathode, a plate and a grid, said grid being adapted to control the plate current of the tube, switching means controlled by the plate current of said tube to initiate and terminate said irradiation, an ionization chamber having a wall adapted to be subjected to said irradiation and comprising at least two electrodes, one of said electrodes being connected to said grid, a source of direct current, a circuit including said source, said ionization chamber, grid and cathode, said source supplying through the cathode a negative charge to the grid-connected chamber-electrode, a plate circuit including said discharge tube, said switching means and at least a portion of said source, and means associated with said first circuit to establish a predetermined decrease oi the potential difference between the cathode and the chamber-electrode which is not connected to the grid, the negative charge of said grid-connected electrode discharging through the ionization chamber during a time interval which determines the duration of the irradiation.

3. A device for automatically determining the duration of irradiation comprising a vacuum discharge tube having a cathode, an incandesclble plate and a grid, said grid being adapted to control the plate current or the tube, switching means controlled by the plate current of said tube to initiate and terminate said irradiation, a source of direct current having a positive connection point and at least two negative connection points of diflerent potential, an ionization chamber having a wall adapted to be subjected to said irradiation and comprising two electrodes, one chamber-electrode being connected to said grid and the other chamber-electrode being connected to the positive connection point, and a changeover device to connect at will the cathode to either one of said two negative connection points.

4. A device for automatically determining the duration of irradiation comprising a vacuum discharge tube having a cathode, a plate and a grid, switching means controlled by the plate current of said tube to initiate and to terminate said irradiation, an ionization chamber having a wall adapted to be subjected to said irradiation and comprising two electrodes, a lead connecting one of said chamber-electrodes to said grid. a member connecting said ionization chamber with said discharge tube and air-tightly surrounding Said lead, and means including a source oi direct current to establish a potential difference between the cathode and the other chamber electrode whereby the cathode is negative with re spect to said latter electrode and to establish a predetermined reduction of said potential diiierence.

5. A device for automatically determining the duration of irradiation comprising a vacuum dischargetube having a cathode, a plate and a grid, switching means controlled by the plate current of said tube to initiate and to terminate said irradiation, an ionization chamber having a wall adapted to be subjected to said irradiation and comprising two electrodes, a lead interconnecting one oi said chamber electrodes and said grid, an insulating tube connecting said discharge tube and said chamber, said insulating tube being exhau'sted and air-tightly enclosing said lead, and means including a source of direct current to establish a potential difference between the cathode and the other electrode whereby the cathode is negative with respect to said latter electrode and toestablish a predetermined reduction oi said potential difference.

6. An X-ray responsive ionization device comprising a sealed envelope having an insulating portion forming an electrode chamber, an insulating neck portion having a considerably smaller cross-sectional area than that of said chamber, and an insulating re-entrant portion connected to said neckportion and extending into said electrode chamber, a gaseous filling within said envelope and ionizable by X-rays, an electrode within said chamber and supported from said re-entrant portion, a second electrode within said chamber and surrounding said first electrode in spaced relationship and supported from a point on one 01 the other of said insulating portions, and a member of conductive material forming part of said neck portion and intercepting the leakage path over the envelope between said point and said re-entrant portion.

7. A device for automatically determining the duration of irradiation, comprising a vacuum discharge tube having an incandesclble cathode, a plate, and a grid to control the plate current of the tube, switching means controlled by the tive charge to said grid-connected electrode, and means associated with said source to establish a predetermined decrease in the potential difference between said cathode and said second electrode, the negative charge of said grid-connected 5 electrode discharging through said chamber during a time interval which determines the duration of the irradiation.

ANTONIUS WILHELMUS VINGERHOETS.

CERTIFICATE OF CORRECTION.

Patent No. 2,122,222.

June 2 19 8.

ANTONIUS WILHELMUS VINGERHOETS.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 1 second column, line 2, claim}, for the words "a cathode, an incandescible" read an incandescible cathode, a; and that the said Letters Patent should be read with this correction thereinvthat the same may conform to the record of the case in the Patent Office.

Signed and sealed this lrth day' of October, A. D. 1958.

(Seal) Henry Van Arsdale Acting, Gonnnissioner of Patents.

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